Field of the Invention
The present invention relates to an epoxy resin-free coating system based on thermoplastic polyesters, into which a special additive resin is incorporated as a promoting component.
For a long time now, coating materials based on thermoplastic polyesters have been produced and marketed for a very wide variety of applications. The common method of production is to incorporate the adjuvants required for the intended application, such as color pigments, fillers, stabilizers, leveling agents, luster agents and/or other additional and auxiliary substances, into the polyester in compounding extruders or kneading apparatus. The compounded formulation (compound) can then be applied as a solution, film, melt, powder or in some other form in the sphere of use envisaged.
A technique which has proven particularly suitable in practice is the powder coating of metals by the fluidized-bed sintering or electrostatic method. In the fluidized-bed sintering method, hot metal parts are coated by dipping them into an air-fluidized bed of the pulverulent coating material. In the course of this procedure the thermoplastic melts onto the surface and, after cooling, forms a protective coating. In electrostatic coating, powder is sprayed by means of compressed air onto the metal part, where it remains adhered through the application of a high direct current voltage. Subsequently, a coating is obtained likewise by melting the applied thermoplastic in an oven or by use of a gas flame, for example. The coating material can also be employed in another way--as a melt or film, for example.
Despite the fact that the preparation of thermoplastic coating powders is relatively complex, because of the cold grinding operation necessary with liquid nitrogen, it has been possible to maintain and even increase the market share of these materials in the face of competition from wet coating materials and reactive systems. Environmental factors are critical here, since coating powders operate entirely without solvent. In addition, purely thermoplastic coating materials have processing advantages over reactive systems, since they are melted on a surface purely by the application of heat and cured by cooling. Because of this they are suitable, inter alia, for extremely rapid coating units operating with downstream further processing, for which reactive systems are generally too slow. By their very nature, reactive systems require chemically reactive components and hardeners, which are often toxicologically objectionable. Moreover, a proportion of residual monomer remains in the coating after the reaction.
These are also the reasons why the use of thermoplastic systems is preferred in connection with the weld-seam protection of welded preserve cans. In the can, the can seam is reliably protected by thermoplastic polyesters against any ingredient, and hence preserves the contents unchanged.
The necessary coating properties, such as adhesion, elasticity and coating sterilizability, are not provided by the thermoplastic polyester alone. Marked improvements have only been obtained, rather, by the admixing the polyester with the appropriate additives. Through the effect of corresponding formulations the properties have been optimized to the extent where, today, coatings of thermoplastic polyesters have attained very good service properties.
The state of the art technique of promoting adhesion is to incorporate epoxy resin additives into the thermoplastic polyesters. In this instance the epoxy resin is not, as is usually the case, chemically crosslinked, but is merely mixed with the polyester, since the polyesters commonly employed do not possess any notable reactive groups. The activity of the epoxy resin additive in terms of the coating properties is excellent and, therefore, makes it possible to comply fully with all requirements placed on the coating system.
Where the coating system is employed in the food packaging sector, the relevant regulations must be met, such as FDA 175.300 (USA), Directive 90/128(EU), and the like. It is here, in particular, that thermoplastic systems, as already stated, have decisive advantages over other systems.
In the recent past epoxy resins employed as coating material in the food packaging sector have come under close public scrutiny. Traces of Bisphenol A diglycidyl ether (BADGE) from epoxy resin coating materials leach out of the interior coatings of preserve cans and are absorbed into the human organism together with the food. In oil-containing fish cans, indeed, inadmissibly high concentrations of BADGE, which leach from the interior coating, have been measured. BADGE is now suspected on intake into the human organism of having brought about carcinogenic and estrogenic effects. A consequence of this is that the Scientific Committee on Foods (SCF) of the European Union (KU) in June 1996 undertook a toxicological reassessment of BADGE. Accordingly, although there is as yet no firm evidence of a carcinogenic effect, the German Federal Institute for Consumer Health Protection and Veterinary Medicine (BgVV) called, in a press release in November 1996, for technological measures to minimize the contamination of foods. BADGE contents of more than 1 mg/kg are classified accordingly as objectionable on health grounds.
Customary thermoplastic coating systems include in their formulation a small percentage of epoxy resin, which of course likewise includes a certain amount of residual monomer (BADGE). As a consequence, the thermoplastic polyester coating system, which in principle is ecologically and toxicologically flawless, has still come in for criticism because of the epoxy resin additive.